First-order SQUID gradiometer with electronic subtraction for magnetocardiography

The paper presents theoretical and experimental results of the research concerning the design and properties of a 1^st-order SQUID gradiometer with electronic subtraction and the recording of the first magnetocardiogram (MCG) performed in Romania, at the Bioelectromagnetism Laboratory of the Faculty of Biomedical Engineering. The MCG recording was possible by using a complex installation composed from a non ferromagnetic shielded room placed in the centre of the large (4 × 4 × 4) m, triaxial Helmholtz coils system. The Helmholtz system ensures the static compensation of the geomagnetic field and the active shielding too, by using the magnetic field negative feedback. Magnetocardiography (MCG) is a noninvasive and risk-free technique for contactless surface mapping of the magnetic fields generated by the electrical activity of the heart. The difficulty of recording of a magnetocardiogram is determined by the very low value of biomagnetic field (10^-15-10^-12)T. Even the strongest of such biomagnetic fields, the one produced by the human heart, barely reaches 100 pT outside the thorax, less than a millionth of the geomagnetic field value and orders of magnitude weaker than environmental magnetic noise fields. In this context, the detection and recording of these fields require an extremely sensitive device such as SQUID biomagnetometer and special conditions. Performances of the installation are presented.